The 1977 Oldsmobile Toronado was one of the first cars to have a microprocessor-based engine control unit. Since then the importance of electronics in cars, trucks, tractors and other vehicles has increased dramatically. Today, even low-end cars have dozens of microprocessors executing tens of millions of lines of software code to control ignition timing, anti-lock braking, airbag deployment, GPS navigation, etc. Aftermarket electronics for navigation, autopilots, asset tracking devices and myriad other applications are seemingly everywhere as people expect modern digital convenience on the go. Microprocessors, GPS receivers, communications radios, and other electronics, versatile as they are, require some protection in an automotive electrical environment, however.
Consider for example a dozer operator in Brazil who needs to start a dozer with a dead battery. Lacking jumper cables, he starts another dozer that has a good battery and then removes the battery with the dozer engine running. He uses the good battery to start the first dozer and puts the dead battery in the dozer that's already running to charge it. This inadvisable procedure (removing a battery with engine running) produces an alternator load dump that puts hundreds of volts across the dozer's nominally 12-volt electrical system and instantly destroys unprotected electronic equipment.
Or imagine a jeepney driver in Manila whose vehicle is equipped with an asset tracking device that sends its position back to a central office. The tracking box is mounted near the roof and connected to the vehicle electrical system by wires that run under the driver's seat. Unfortunately, due to poor installation and flimsy insulation, the weight of the driver compresses the cable under the seat and shorts it out whenever the jeepney hits an especially hard bump. Normally the short does not last long enough to blow a fuse, but sometimes it does. The jeepney company wonders why the fuses blow so often.
FIG. 1 is a schematic diagram for a conventional overvoltage protection circuit used in automotive systems. In the circuit, VOUT is protected from excess voltage at VIN, and fuse F1 provides overcurrent protection. The operation of the circuit is as follows.
A trigger voltage is defined by VTR=VREF+VBE. Here VREF is the breakdown voltage of Zener diode D1 and VBE(≈0.6 V) is the base-emitter voltage of transistor Q1. D1 is called the “reference diode” because it sets the input voltage above which the circuit cuts off the output. If VIN is less than VTR then VOUT is equal to VIN. If VIN is greater than VTR then VOUT is zero.
In more detail, when VIN<VTR: no current flows through D1; Q1 is cut off; the gate voltage of MOSFET pass transistor Q2 is at ground; and Q2 is on. On the other hand, when VIN>VTR: current flows through D1; the base current of Q1 is (VIN−VTR)/R2; Q1 is saturated; the gate voltage of Q2 is (VIN−VCE); and Q2 is off. (VCE is the collector-emitter voltage of Q1.)
Zener diode D2 protects Q2 by preventing the gate voltage from becoming too great. R1 keeps Q1 turned off when VIN is less than VTR. R2 sets the base current of Q1 when D1 turns on. Q1 is called the “control transistor” because it supplies current to R4 (called the “bias resistor”) to raise the gate voltage of Q2, turning Q2 off.
A circuit like that shown in FIG. 1 will save the dozer driver from frying his GPS, but it won't stop the jeepney company from having to buy more fuses. What is needed is an overvoltage/overcurrent protection circuit that doesn't require operator assistance. The circuit should clamp voltage spikes, disconnect the load whenever a short circuit occurs, and reconnect the load when the short circuit is removed.